Effective scheduling of irrigation is important in modern irrigated agricultural production. Such scheduling is especially important in decisions related to maximizing yields in areas where irrigation water supplies are limited. Even if irrigation water supplies are plentiful, effective irrigation scheduling is needed to make the most efficient use of energy and water while avoiding problems associated, for example, with over-watering such as erosion and leaching.
For many years the scheduling of irrigation of an agricultural area has been performed in a relatively unosphisticated manner such as periodically for a set period of time. Such procedure, however, affords little assurance that the crop is not being damaged by over- or under-watering.
In an effort to provide for more effective and efficient scheduling of irrigation, systems, and methods have been devised which base irrigation scheduling on measurement of physical properties such as soil moisture content, temperature, humidity and water flow. Such systems consequently use one or more probes or sensors to measure such physical properties. In some of these systems, the probes or sensors are connected to an electrical or electronic device such as a microprocessor which effects analysis of the sensed data as by comparison to a given standard. Hence, irrigation might be initiated by such systems when, for example, the soil moisture content falls below a specified level considered to be the minimum soil moisture content for desired production of the crop being grown.
For many years the concept of using canopy temperature to detect the onset and duration of plant water stress has been known (Tanner, 1963; Weigand and Namken, 1966; Ehrler and van Bavel, 1967; Astin and van Bavel, 1972; Bartholic et al, 1972; and Ehrler, 1973). When a leaf is freely transpiring, the cooling properties of the evaporating water keep the leaf temperature below that of the air. When plant water intake becomes deficient as when soil moisture content is low, the heat load of the leaf builds up because convection and thermal radiation are insufficient to dissipate the heat load. Thus, the leaf temperature will approach and often rise above air temperature when soil moisture content is low.
Several researchers have reported theoretical concepts which make use of canopy/air temperature difference in describing the effects of water stress on crop yields. One of these concepts uses only the canopy/air temperature difference (Idso et al, 1977), whereas another also incorporates the vapor pressure deficit (Idso et al, 1981). A third concept adds the radiation load and is based on the energy balance of the leaf (Jackson et al, 1981). These three concepts respectively rely on what has been termed the stress degree day (SDD), the water stress index (WSI) and the crop water stress index (CWSI). The first two indices are empirical simplifications of the third and all three indices were utilized in arid regions. In a later work (Keener and Kircher, 1983), it was shown that in arid environments all three indices appear to work equally well as a describer of yield losses due to water stress whereas only the third index may be useful in describing yield losses in humid environments.
Respecting the scheduling of irrigation, it has been suggested that the SDD index can be utilized to schedule irrigation (Idso et al, 1977). Others also have developed techniques based on canopy temperature to schedule irrigation. As reported in Slack et al (1981) canopy temperatures were used to schedule irrigation of maize in a sub-humid region. In a later paper (Slack et al, 1983), two approaches were investigated as a means of simplifying irrigation scheduling procedures using crop canopy/air temperature difference, one of which involved a hand-held automated instrument package including an infrared thermometer, a net radiometer or pyranometer, and a psychrometer interfaced with a microcomputer for collection and analysis of data.
In Clawson and Blad (1982), it was concluded that canopy temperature variability can be used to signal the onset of plant water stress in maize but that the severity of the stress is better indicated by the magnitude of the elevation of the average canopy temperature above that of a well watered reference plot. Geiser et al (1982) showed that the approach of Slack et al (1981) could reduce the water applied to maize plots (as compared to irrigation scheduling by a checkbook method or resistance blocks (without reducing yields).
Accordingly, a few have used canopy temperatures to schedule irrigation whereas others have alluded to the possibility of using canopy temperature as an irrigation scheduling tool. Generally, the art relating to irrigation scheduling based on canopy temperatures is in its infancy and is begging for more effective, practical and efficient systems and methods for scheduling irrigation.